Electrode and preparation thereof

ABSTRACT

Preparation of an electrode comprising a substrate of a valve metal or of an alloy thereof having similar properties thereto and a coating thereon comprising at least an outer layer of an electrocatalytically-active material which comprises an oxide of at least ruthenium and an oxide of at least one non-noble metal by a one-step coating process which comprises the vapor phase deposition of a mixture of at least ruthenium and/or oxide thereof and at least one non-noble metal or oxide thereof onto the substrate. The outer layer is of substantially uniform thickness, the contours thereof are at least substantially the same as the contours of the substrate underlying it and the electrode affords an increased surface area for a given mass of catalyst and a more efficient use of catalyst to obtain a given thickness thereof.

FIELD OF THE INVENTION

This application is a 371 of PCT/GB94/01718 filed on Aug. 4, 1994.

This invention relates to an electrode for use in an electrolytic cell,particularly to an electrode for use as an anode in an electrolyticcell, especially in an electrolytic cell in which in operation chlorineis evolved at the anode, although use of the anode of the invention isnot restricted to electrolyses in which chlorine is evolved, and to amethod for the preparation of the electrode.

BACKGROUND OF THE INVENTION

Electrolytic processes are practiced on a large scale throughout theworld. For example, there are many industrial processes in which wateror an aqueous solution is electrolyzed, for example, an aqueous solutionof an acid or an aqueous solution of an alkali metal chloride. Aqueousacidic solutions are electrolyzed in, for example, electrowinning,electrotinning and electrogalvanizing processes, and aqueous alkalimetal chloride solutions are electrolyzed in the production of chlorineand alkali-metal hydroxide, alkali metal hypochlorite, and alkali metalchlorate. The production of chlorine and alkali metal hydroxide ispracticed in electrolytic cells which comprise a mercury cathode or inelectrolytic cells which comprise a plurality of alternating anodes andcathodes, which are generally of foraminate structure, arranged inseparate anode and cathode compartments. These latter cells alsocomprise a separator, which may be a hydraulically permeable porousdiaphragm or a substantially hydraulically impermeable ion-exchangemembrane, positioned between adjacent anodes and cathodes therebyseparating the anode compartments from the cathode compartments, and thecells are also equipped with means for feeding electrolyte to the anodecompartments and if necessary liquid to the cathode compartments, andwith means for removing the products of electrolysis from thesecompartments. In a cell equipped with a porous diaphragm, aqueous alkalimetal chloride solution is charged to the anode compartments of thecell, and chlorine is discharged from the anode compartments andhydrogen and cell liquor containing alkali metal hydroxide aredischarged from the cathode compartments of the cell. In a cell equippedwith an ion-exchange membrane aqueous alkali metal chloride solution ischarged to the anode compartments of the cell and water or diluteaqueous alkali metal hydroxide solution to the cathode compartments ofthe cell, and chlorine and depleted aqueous alkali metal chloridesolution are discharged from the anode compartments of the cell andhydrogen and alkali metal hydroxide are discharged from the cathodecompartments of the cell.

Electrolytic cells are also used in the electrolysis of non-aqueouselectrolytes and in electrosynthesis.

It is desirable to operate such electrolytic cells at as low a voltageas possible in order to consume as little electrical power as possibleand in such a way that the component parts of the electrolytic cell arelong lasting, i.e. the electrodes in the electrolytic cell should have along lifetime.

In recent years anodes which have been used in such electrolyticprocesses have comprised a substrate of titanium or of an alloy oftitanium possessing properties similar to those of titanium and acoating of an electrocatalytically-active material on the surface of thesubstrate. An uncoated titanium anode could not be used in such anelectrolytic process as the surface of the titanium would oxidize whenanodically polarized and the titanium would soon cease to function as ananode. The use of such a coating of electrocatalytically-active materialis essential in order that the titanium shall continue to function as ananode. Examples of such electrocatalytically-active materials which havebeen used include metals of the platinum group, oxides of metals of theplatinum group, mixtures of one or more such metals and one or more suchoxides, and mixtures or solid solutions of one or more oxides of aplatinum group metal and tin oxide or one or more oxides of a valvemetal, that is one or more oxides of titanium, tantalum, zirconium,niobium, hafnium or tungsten.

Recently it has been suggested in EP 0,437,178 that anodes wherein thecoating comprises mixed oxides of iridium, ruthenium and titanium havingoxide molar ratios of Ti:(Ir+Ru) of less than 1:1 and of Ru:Ir ofbetween 1.5:1 and 3:1 can be prepared from a certain acidic aqueoussolution.

Likewise, it has been suggested in J 59,064788 that electrode coatingscan be prepared by the deposition of certain coatings from organicsolvents onto a substrate followed by heating the coated substrate inoxygen.

SUMMARY OF THE INVENTION

We have now found surprisingly that electrodes for use in electrolyticcells may be prepared by the physical vapor deposition of a mixture ofpowders of (i) ruthenium oxide, (ii) a non-noble metal oxide, e.g. tinoxide, or a valve metal oxide and preferably (iii) a noble metal oxideother than ruthenium oxide (hereinafter referred to for convenience as"second noble metal oxide"), onto a suitable substrate. This method hasthe advantage that it affords a single step coating process for thepreparation of an electrode. Moreover, the durability of the electrodemay be improved by a subsequent heat treatment as is more fillydescribed hereinafter.

The present invention provides a method for the preparation of anelectrode which (a) comprises a substrate of a valve metal or alloythereof and a coating on the substrate which comprises at least an outerlayer having uniform thickness, particularly where prepared by RFsputtering, and of good electrocatalytic activity and (b) when used asan anode in a cell in which chlorine is evolved at an anode has anacceptable overvoltage and often, as is hereinafter more fullydescribed, has high durability.

According to the present invention there is provided a method for thepreparation of an electrode which comprises a substrate of a valve metalor of an alloy thereof and a coating thereon comprising at least anouter layer of an electrocatalytically-active material which comprisesan intimate mixture of ruthenium oxide and at least one non-noble metaloxide which process comprises the step of depositing a mixture of theaforementioned oxides on the substrate by physical vapor deposition(PVD).

Preferably, mixture of oxides in the outer layer of the coating on theelectrode prepared by the process according to the present inventioncontains an oxide of a second noble metal.

DETAILED DESCRIPTION

As examples of PVD may be mentioned inter alia radio frequency (RF)sputtering, sputter ion plating, arc evaporation, electron beamevaporation, dc magnetron, reactive PVD, etc. or combinations thereof.It will be appreciated that where combinations of evaporation techniquesare used in the same evaporation chamber in the PVD system separatetargets may be used, e.g. a ruthenium target and a tin target insteadof, or in addition to, a mixed ruthenium/tin target. By "target" we meanthe material which is vaporized to produce a vapor for deposition on thesubtrate in the PVD system.

The substrate of the electrode comprises a valve metal or an alloythereof Suitable valve metals include titanium, zirconium, niobium,tantalum and tungsten, and alloys comprising one or more such valvemetals and having properties similar to those of the valve metals.Titanium is a preferred valve metal as it is readily available andrelatively inexpensive when compared with the other valve metals.

The substrate may consist essentially of a valve metal or alloy thereof,or it may comprise a core of another metal, e.g. steel or copper, and anouter surface of a valve metal or alloy thereof.

The oxide of the non-noble metal in the outer layer of the coating maybe, for example, a valve metal as hereinbefore described, or cobalt orpreferably tin.

The oxide of the at least one second noble metal, where present in theouter layer of the coating, may be, for example, an oxide of one or moreof rhodium, osmium, platinum or preferably iridium.

The electrode prepared by the process according to the present inventionwhen used as an anode in an electrolytic cell in which chlorine isevolved at the anode, has a low overvoltage acceptable in terms ofchlorine evolution, i.e. less than 100 mV at 3 kA/m². Moreover, we havefound surprisingly that where the oxidic component of the aforementionedouter layer provides more than 30 atomic % of all the components in theouter coating, as measured by X-ray absorption spectroscopy, theelectrode has high durability.

The possibility is not excluded of the coating of the electrodecomprising one or more further layers intermediate the outer layer andthe substrate, but it will be described hereinafter with reference to acoating which consists of only the aforementioned outer layer.

The layers in the coating are described as variously comprising an oxideof ruthenium and an oxide of at least one non-noble metal and preferablyan oxide of at least one second noble metal. Although the various oxidesin the layers may be present as oxides per se it is to be understoodthat the oxides may together form a solid solution in which the oxidesare not present as such. For example, where a layer in the coating,particularly the outer layer, comprises a second noble metal oxide, e.g.iridium oxide, the intimate mixture may be in the form of a solidsolution of, for example, ruthenium dioxide, iridium oxide and tindioxide or a solid solution of two of them mixed with the third. We donot exclude the possibility that a noble metal per se or an alloythereof may be present in the coating.

In general the electrode will be used in the electrolysis of aqueouselectrolytes and although the electrode of the invention is particularlysuitable for use as an anode at which chlorine is evolved the electrodeis not restricted to such use. It may, for example, be used as an anodein the electrolysis of aqueous alkali metal chloride solution to producealkali metal hypochlorite or alkali metal chlorate, or it may be used asan anode at which oxygen is evolved.

The over-voltage and useful working lifetime of the electrode preparedby the method according to the present invention is dependent at leastto some extent on the ratio of the components in the coating on theelectrode and on the thickness therof. The coating will generallycomprise at least 10 mole % in total of oxide of noble metal, i.e.ruthenium and the second noble metal, where present, and at least 20mole % of oxide of non-noble metal.

In general the coating will be present at a loading of at least 5 g/m²of nominal electrode surface, preferably at least 10 g/m². In general itwill not be necessary for the coating to be present at a loading ofgreater than 100 g/m², preferably not greater than 50 g/m². Typically,the thickness of the outer layer of the coating is between 1 and 10μ.

In the method according to the present invention, the chamber in the PVDsystem is charged with oxygen or ozone and an inert gas, preferablyargon.

Where the method according to the present invention is carried out inthe reactive mode, i.e. the target in the PVD system is metallic, theratio of oxygen:argon is greater than 2:1 by volume and preferably is atleast 4:1 by volume.

The specific conditions used in the method according to the presentinvention may be found by the skilled man by simple experiment.

For example, the pressure in the deposition chamber may be in the range10⁻² to 10⁻¹⁰ atmospheres, particularly where the coating comprises amixture of ruthenium oxide, iridium oxide and tin oxide.

We have found that the useful working life of the electrode prepared bythe method according to the present invention may be increased bysubjecting it to a treatment at high temperature of at least 400° C.,typically about 500° C., for at least one hour.

Where the electrode of the present invention comprises an intermediatelayer it may, for example, comprise RuO₂ and an oxide of at least onenon-noble metal. The oxide of the non-noble metal in the intermediatelayer may be, for example, titanium oxide, zirconium oxide, or tantalumpentoxide or oxide of another valve metal. Alternatively, or inaddition, the intermediate layer may comprise an oxide of a non-noblemetal other than a valve metal, and tin is an example of such anon-noble metal.

The structure of the electrode, and of the electrolytic cell in whichthe electrode is used, will vary depending upon the nature of theelectrolytic process which is to be effected using the electrode. Forexample, the nature and structure of the electrolytic cell and of theelectrode will vary depending upon whether the electrolytic process isone in which oxygen is evolved at the electrode, e.g. as in anelectrowinning process, an electroplating process, an electrogalvanisingprocess or an electrotinning process, or one in which chlorine isevolved at the electrode, or one in which alkali metal chlorate oralklai metal hypochiorite is produced, as is the case where aqueousalkali metal chloride solution is electrolyzed. However, as theinventive feature of the present invention does not reside in the natureor structure of the electrolytic cell nor of the electrode there is nonecessity for the cell or the electrode to be described in any detail.Suitable types and structures of electrolytic cell and of electrodes maybe selected from the prior art depending on the nature of theelectrolytic process. The electrode may for example, have a foraminatestructure, as in a woven or unwoven mesh, or as in a mesh formed byslitting and expanding a sheet of valve metal or alloy thereof, althoughother electrode structures may be used.

Prior to deposition of the coating on the substrate the substrate may besubjected to treatments which are also known in the art. For example,the surface of the substrate may be roughened, for example bysand-blasting, in order to improve the adhesion of the subsequentlyapplied coating and in order to increase the real surface area of thesubstrate. The surface of the substrate may also be cleaned and etched,for example by contacting the substrate with an acid, eg with an aqueoussolution of oxalic acid or hydrochloric acid, and the acid-treatedsubstrate may then be washed, e.g. with water, and dried.

According to the present invention there is provided an electrode whichcomprises a substrate of a valve metal or of an alloy thereof and acoating thereon comprising an outer layer of anelectrocatalytically-active material which comprises an intimate mixtureof ruthenium oxide and at least one non-noble metal oxide wherein theouter layer is of substantially uniform thickness and wherein thecontours of the surface of the outer layer are at least substantiallythe same as the contours of the substrate immediately underlying it.

Such an electrode affords the advantages of an increased surface areafor a given mass of catalyst and the more efficient use of theelectrocatalytically-active material to obtain a minimum thicknessthereof.

The contour of the surface of the outer layer of electrode coatingsprepared by processes known in the art, for example by the method ofOnuchukwa and Trasatti, J Applied Electrochemistry, 1991, Vol. 21,858,are non-uniform and tend to deviate from the contour of the surface ofthe substrate immediately underlying it, for example the outer layer isformed with thicker projections and shallower depressions.

We have found that where the outer layer of the coating of the electrodeaccording to the present invention comprises a mixture of tin, iridiumand ruthenium oxides it is often in the form of small particles,typically of less than 100A, of a iridium/ruthenium intermetallic,containing 70-100% of the iridium and 40-80% of the ruthenium, in amixture of a poorly crystalline tin oxide/iridium oxide/ruthenium oxidemixture.

The present invention is illustrated by reference to the accompanyingdrawing which represents, by way of example only, a micrograph of anelectrode according to the present invention prepared by the method ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing: FIG. 1 is a micrograph of a cross-section of anelectrode prepared in Example 1.

In FIG. 1, (1) is the electrode coating, (2) is the electrode substrateand (3) is the base on which the electrode was mounted for preparing themicrograph.

From FIG. 1, it can be seen that the electrode coating (1) is of uniformthickness and that the contour of the surface thereof is substantiallythe same as the contour of the substrate immediately underlying it (2).

The present invention is further illustrated by the following Examples.

EXAMPLES 1-2

These Examples illustrate the preparation of electrodes by the methodaccording to the present invention using RF sputtering.

A powder for coating an electrode was prepared by dissolving RuCl₃ (7.5g), H₂ IrCl₆ (3.2 g) and SnCl₂ (13.5 g) in propan-2-ol (200 mls). Thesolution was evaporated to dryness under vacuum. Sodium nitrate (40 g)was added to the residual solid and the mixture was heated to 450° C. inair for 2 hours. The heat-treated mixture was washed with hot water thencold water and dried at 150° C. The dried solid was ground by glassbeads and a portion of the ground solid was collected by sieving through+45, -106 standard meshes. In the collected portion, the weight ratio ofRu:Ir:Sn was 1.6:1:3.7.

Two samples of titanium sheet were cleaned by contacting them withacetone, the cleaned samples were dried, etched for 8 hours in 10% w/voxalic acid at 90° C. and etched further immediately prior to coating.

The samples were separately mounted on stainless steel plates (held witha nickel foil mask) and disposed in the PVD system which was allowed topump down overnight.

In Example 1, the pressure in the PVD system was adjusted to 6×10⁻² mbarby controlling the argon flow, the powder target was presputtered for 5hours at 500 W incident RF power, the target shutter was removed and thesample was coated for 20 hours. A nominal coating thickness of 2 μm wasobtained.

In Example 2, the pressure in the PVD system was adjusted to 5×10⁻¹ mbarby controlling the argon flow, the already conditioned powder targetfrom Example 1 was presputtered for 2 hours at 500 W incident RF power,the target shutter was removed and the sample was coated for 20 hours. Anominal coating thickness of 2 μm was obtained.

The coated titanium samples from Examples 1 and 2 were separatelyinstalled in electrolytic cells as an anode and spaced from a nickelcathode. The anode was subjected to an accelerated test in which anaqueous solution containing 20 weight % NaCl and 20 weight % NaOH waselectrolyzed at a constant current density of 20 kA/m² and at atemperature of 65° C.

The electrode was tested for chlorine-producing activity, i.e. chlorineoverpotential, by measurement of the potential decay curve as a constantcurrent is interrupted.

In a Comparative Test, an anode comprising a coating of RuO₂ :IrO₂ :SnO₂in weight ratio 25:10:65 was prepared by so-called spray-baking. Thespray-baked anode was prepared by: (i) rolling a bottle containing RuCl₃(1.5 g) in pentanol (30 cm³) for 8 hours, adding H₂ IrCl₆ (0.63 g) tothe solution formed thereby and rolling for 2 hours; (ii) addingstannous octoate (6.2 g), 4-tert-butyl catechol (0.15 g) and2,5-di-tert-butyl quinol (0.15 g) to the solution formed in (i) androlling for 1 hour; (iii) coating a titanium substrate by applying aportion of the solution from (ii) thereto by brush; (iv) drying thecoated substrate by heating for 10 minutes at 180° C. and (v) baking thedried coated substrate at 510C. for 20 minutes. Steps (iii)-(v) wererepeated until a coating on the titanium substrate of the desiredthickness was obtained.

Samples from Examples 1 and 2 were post heat-treated at 500° C. for 2hours in flowing air. The useful working lives of the post heat-treatedsamples and of the anode from the Comparative Test were determined.

The useful working life-time of the electrode is defined as the timetaken for the anode to cathode voltage in the aforementioned solution torise 2 V above its starting value. The results are shown in Table 1 fromwhich it can be seen that anodes prepared by the method according to thepresent invention have good activity and good durability.

                  TABLE 1    ______________________________________            Chlorine overpotential at                            Working life-time of    Example 3 kAm.sup.-2 (mV)                            heat-treated anode (hours)    ______________________________________    1       85              >360    2       55              >380    CT      60              264    ______________________________________     CT: Comparative Test

EXAMPLE 3

This Example illustrates the good long term performance of an electrodeprepared by the method according to the present invention in theproduction of chlorine.

The procedure of Example 1 was repeated and the heat-treated electrodewas installed as an anode in a laboratory membrane cell containing aNafion (RTM) 90209 membrane, nickel cathode, anolyte of saturated brineat 90° C. and catholyte of 32% sodium hydroxide at 90° C. The cell wasoperated at 3 kAm⁻².

Cell voltage data obtained therefrom is shown in Table 2 from which itcan be seen that the electrode has a good long-term performance.

                  TABLE 2    ______________________________________    Time on load  Cell voltage    (days)        (volts)    ______________________________________    0             3.3    127           3.4    ______________________________________

Measurements of RuO₂ content of the electrocataixtically-active coatingby X-Ray fluoresence (XRF) analysis revealed low coating losses underthe aforementioned operating conditions as shown in Table 3.

                  TABLE 3    ______________________________________    Time on load  Loading RuO.sub.2    (days)        (g/m.sup.2)    ______________________________________    0             10.63    373           10.14    ______________________________________

EXAMPLES 4-5

These Examples illustrate electrodes prepared by the method according tothe present invention using arc-evaporation.

Ruthenium and tin metal powders, in weight ratio 3:7, were mixed andhot-pressed to form a PVD target. The PVD target was disposed in an arcevaporation system and a mixture of oxygen and argon was passed throughthe system.

Material was evaporated from the target and deposited onto titaniumsubstrates which had been etched by the procedure described in Example1.

The conditions used in the arc evaporation system are shown in Table 4.

                  TABLE 4    ______________________________________                 Flow Rates    Arc Current  (sccm)    Substrate Chamber Pressure    Example           (A)       O.sub.2                            Ar   Bias (Volts)                                         (mbar)    ______________________________________    4      35        80     10   -50     0.003    5      20        40     10   -50     0.003    ______________________________________

The chlorine overpotential of the electrode of Example 4 was found to be85 mV at 3 kAm⁻², measured by the so-called "current interrupt method"in which a constant current was interrupted, the potential decay curvewas displayed on an oscilloscope from which the overpotential could beread directly.

I claim:
 1. A method for the preparation of an electrode comprising asubstrate of a valve metal or of an alloy thereof and a coatingcomprising at least an outer layer of an electrocatalytically-activematerial which comprises an intimate mixture of ruthenium oxide and atleast one non-noble metal oxide which method comprises the Steps of:(A)deposition a mixture comprising said oxides onto the substrate byphysical vapor depositing (PVD) in the deposition chamber in a PVDsystem charged with oxygen or zone and an inert gas with the provisionthat where the reaction is carried out in the reactive mode, the ratioof oxygen or zone to argon is greater than 2:1 by volume; and (B)subjecting the coated substrate prepared in step (A) to heat treatmentat a temperature of at least 400° C. for at least 1 hour.
 2. A method asclaimed in claim 1 wherein the physical vapor deposition comprises radiofrequency sputtering, sputter ion plating, arc evaporation, electronbeam evaporation, dc magnetron evaporation, or reactive physical vapordeposition.
 3. A method as claimed in claim 1 wherein the pressure inthe deposition chamber in the PVD system is in the range of from 10⁻² to10⁻² atmospheres.
 4. A method as claimed in claim 1 wherein thenon-noble metal is tin.
 5. A method as claimed in claim 1 wherein theintimate mixture comprises ruthenium oxide, a non-noble metal oxide andan oxide of a second noble metal.
 6. A method as claimed in claim 5wherein the second noble metal is iridium.
 7. A method as claimed inclaim 6 wherein the coating comprises a mixture of RuO2, IrO2 and SnO2.8. A method as claimed in claim 1 or 5 wherein the coating comprises atleast 10 mole % of oxide of noble metal and at least 20 mole % of oxideof non-noble metal.
 9. A method as claimed in claim 1 wherein the heattreatment in Step (B) is carried out at about 500° C.